Metabolism: Absorptive vs. Postabsorptive States
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Questions and Answers

During prolonged fasting, which of the following statements about energy sources is accurate?

  • Muscles primarily use glucose as their main energy source during fasting.
  • Fatty acids become the primary fuel source for the brain.
  • The brain exclusively relies on amino acids for energy.
  • After 2-3 weeks, ketone bodies replace glucose as the major fuel for the brain. (correct)

What occurs in the liver during well-fed conditions regarding gluconeogenesis?

  • Gluconeogenesis is only activated in response to low insulin levels.
  • Gluconeogenesis is decreased as glucose levels are adequate. (correct)
  • Gluconeogenesis is significantly increased to supply glucose.
  • Gluconeogenesis remains unchanged regardless of nutritional state.

Which statement accurately describes the role of adipose tissue during fasting?

  • There is an increased degradation of fat, releasing more fatty acids. (correct)
  • Fatty acids storage becomes the primary focus in adipose tissues.
  • Adipose tissue synthesizes more triacylglycerol during fasting.
  • Glucose uptake is enhanced despite low insulin levels.

Which of the following processes is primarily activated in muscle tissue during fasting?

<p>Rapid breakdown of muscle protein to provide amino acids for gluconeogenesis. (C)</p> Signup and view all the answers

What is the effect of cortisol on gluconeogenesis during the postabsorptive state?

<p>Basal cortisol levels stimulate gluconeogenesis. (A)</p> Signup and view all the answers

What characterizes the metabolic response in the absorptive state compared to the fasting state?

<p>Enhanced protein synthesis and increased TAG synthesis (B)</p> Signup and view all the answers

Which hormone primarily regulates glucose metabolism during the fasting state?

<p>Glucagon (C)</p> Signup and view all the answers

What is the main fuel source utilized by tissues during the absorptive state?

<p>Glucose from digested carbohydrates (D)</p> Signup and view all the answers

During fasting, what is the body's priority for sustaining energy levels in essential tissues?

<p>Maintaining adequate plasma glucose levels (C)</p> Signup and view all the answers

Which statement best summarizes the hormonal changes that occur during the transition from the absorptive state to fasting?

<p>Glucagon and epinephrine secretion increases while insulin decreases (B)</p> Signup and view all the answers

Flashcards

Absorptive State

The period after eating, typically 2-4 hours after a meal.

Postabsorptive (Fasting) State

The period when no food is consumed, typically several hours after the last meal.

Insulin: What is it?

A hormone that promotes uptake and storage of glucose, dominant during the absorptive state.

Glucagon: What is it?

A hormone that promotes glucose release and fat breakdown, dominant during the postabsorptive state.

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Glucose: Fuel Source in the Absorptive State

The primary fuel source during the absorptive state, derived from the digestion of carbohydrates.

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Glycogenolysis

The process of breaking down glycogen into glucose. It is used to maintain blood glucose levels when the body is fasting or not consuming enough carbohydrates.

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Gluconeogenesis

The process of creating new glucose from non-carbohydrate sources, primarily from amino acids and glycerol. It is crucial for maintaining blood glucose levels during fasting.

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Glycogenesis

The process of storing excess glucose in the liver in the form of glycogen. This serves as a readily available source of energy for the body.

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Brain fuel

The primary fuel used by the brain. Although the brain can use ketone bodies during prolonged fasting, glucose is its preferred source of energy.

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Lipolysis

The process of breaking down fats (triacylglycerol) into fatty acids and glycerol. This occurs during fasting or when energy demands are high. Occurs mainly in adipose tissue.

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Study Notes

Feeding (Absorptive) State

  • Occurs 2-4 hours after a meal
  • Plasma glucose, amino acids, and triacylglycerols (TAG) increase transiently
  • Pancreatic islet cells release more insulin and less glucagon
  • High insulin-to-glucagon ratio triggers anabolic processes
  • TAG and glycogen synthesis increase, replenishing fuel stores
  • Protein synthesis also increases
  • Virtually all tissues utilize glucose as fuel
  • Liver, adipose tissue, skeletal muscle, and brain metabolism change significantly

Postabsorptive (Fasting) State

  • Begins after the absorptive period
  • Food intake is absent
  • Plasma glucose, amino acids, and TAG levels decrease
  • Insulin secretion decreases
  • Glucagon and epinephrine secretion increase
  • Decreased insulin/counter-regulatory hormone ratio leads to catabolic processes
  • TAG, glycogen, and protein breakdown occur
  • Two priorities: maintain blood glucose levels and mobilize fats
  • Fatty acids and ketone bodies become energy sources

Feature Comparison

Feature Feeding (Absorptive) State Postabsorptive (Fasting) State
Definition Period shortly after a meal (2-4 hours) Period without food intake
Primary Hormone Insulin Glucagon, epinephrine
Fuel Source Glucose from carbohydrates Stored glycogen, fats, and ketone bodies
Glucose Metabolism Increased glucose uptake and utilization by most tissues Glycogenolysis and gluconeogenesis to maintain blood glucose levels
Liver Activity Glycogenesis, lipogenesis Glycogenolysis, gluconeogenesis
Fat Metabolism Lipid synthesis and storage Lipolysis, increased fatty acid oxidation, ketone body synthesis
Protein Metabolism Protein synthesis Protein breakdown (amino acids for gluconeogenesis)
Brain Fuel Glucose Initially glucose, then ketone bodies
Tissues Using Glucose Almost all tissues Primarily brain and red blood cells; other tissues use fatty acids and ketones
Fatty Acid Usage Low High
Ketone Bodies Not produced or used significantly Produced in the liver for alternative fuel
Energy Storage Excess nutrients stored as glycogen, fat, and protein Stored energy reserves mobilized for energy

Liver in Fed vs. Fasting (Carbohydrate Metabolism)

  • Fed (Well-fed):*

  • Increased glucose uptake by hepatocytes (GLUT 2)

  • Increased glucose phosphorylation

  • Increased glycogenesis

  • Increased glycolysis (after carbohydrate-rich meal)

  • Gluconeogenesis and glycogenolysis are decreased

  • Fasting:*

  • Increased glycogen degradation

  • Increased glucose synthesis

Liver in Fed vs. Fasting (Fat Metabolism)

  • Fed:*

  • Increased fatty acid synthesis

  • Increased triacylglycerol synthesis (VLDL)

  • Fasting:*

  • Increased fatty acid oxidation

  • Increased ketone body synthesis

Liver in Fed vs. Fasting (Protein Metabolism)

  • Fed:*

  • Increased protein synthesis

  • Increased amino acid degradation

  • Fasting:*

  • Protein breakdown increases, providing amino acids for gluconeogenesis. (This is particularly significant in early stages)

Adipose Tissue: Energy Storage Depot

  • Fed (Well-fed):*

  • Increased glucose transport into adipocytes (GLUT 4)

  • Increased glycolysis (for glycerol-3-phosphate in TAG synthesis)

  • Fasting:*

  • Decreased glucose transport (due to low insulin)

  • Increased fat degradation

  • Increased release of fatty acids

Resting Skeletal Muscle (Carbohydrate)

  • Fed:*

  • Increased glucose transport into muscle cells (GLUT 4)

  • Increased glycogen synthesis

  • Fasting:*

  • Switches from glucose to fatty acids/ketone bodies as fuel

  • Glucose transport and metabolism are depressed due to low insulin

Resting Skeletal Muscle (Protein)

  • Fed:*

  • Increased protein synthesis

  • Increased uptake of branched-chain amino acids

  • Fasting:*

  • Rapid muscle protein breakdown in the first few days, providing amino acids for hepatic gluconeogenesis

  • Rate of muscle proteolysis declines after the first few days as the brain starts using ketone bodies as a fuel

Resting Skeletal Muscle (Fat Metabolism)

  • Fed:*

  • Fatty acids are less important, glucose is the primary fuel

  • Fasting:*

  • Muscle uses fatty acids from adipose tissue and ketone bodies in the first 2 weeks

  • Primarily uses fatty acids after ~3 weeks

Brain

  • Primarily uses glucose as fuel
  • No glycogen stores
  • Dependent on blood glucose levels
  • Blood glucose below 40 mg/100 ml impairs cerebral function
  • In prolonged fasting, ketone bodies replace glucose as the main brain fuel

Kidney and Long-Term Fasting

  • Contributes to gluconeogenesis (50%)
  • Compensates for ketoacidosis
  • Corrected pH with glutaminase and ammonia

Effects of Cortisol on Organic Metabolism

  • Permissive for gluconeogenesis and lipolysis in the post-absorptive state
  • Increased cortisol leads to
    • Increased protein catabolism
    • Increased gluconeogenesis
    • Decreased glucose uptake by muscle and adipose tissue
    • Increased triglyceride breakdown
    • Net result: increased plasma amino acids, glucose, and free fatty acids

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Description

This quiz explores the metabolic processes during the absorptive and postabsorptive states following a meal. It covers the hormonal changes, nutrient utilization, and energy sources involved in each state. Test your understanding of how the body transitions from an anabolic to a catabolic state.

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